White-throated Sparrows winter in the southeastern US, breeding across Canada and parts of the northern United States. During early spring, birds store large amounts of fat as an energy source during migration. Reproductive development occurs during spring. Following June breeding, gonads regress and remain small until next spring. These events (fattening and gonad size increase) and migratory restlessness and orientation towards north or south are all affected by prolactin from the pituitary and corticosteroids from the Adrenal cortex.
A. H. Meier injected corticosteroids into White-throated Sparrows at time zero. Prolactin given at 12 hours results in: rise of fat stores, rise in gonad size, rise in migratory restlessness and a rise in locomotor activity towards north (= spring migrant). Prolactin given at 4 hours results in arise in fat stores, rise in migratory restlessness, no gonad increase, and an orientation to south (= fall migrant). Prolactin at 8 hours decreases fat stores, decreases locomotor activity, and decreases gonad weight (= winter bird).
This does not explain how the bird knows which way is north or South.